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A and B react together to form an equilibrium mixture - AQA - A-Level Chemistry - Question 9 - 2020 - Paper 2
Question 9
A and B react together to form an equilibrium mixture. A(aq) + 2B(aq) ⇌ C(aq) An aqueous solution containing 0.25 mol of A is added to an aqueous solution containi... show full transcript
Worked Solution & Example Answer:A and B react together to form an equilibrium mixture - AQA - A-Level Chemistry - Question 9 - 2020 - Paper 2
Step 1
Calculate the amount of A and the amount of B, in moles, in the equilibrium mixture.
Answer
Let the initial amounts of A and B be 0.25 mol each.
Using the equation:
When equilibrium is reached, we have:
Amount of C at equilibrium = 0.015 mol.
- The change in concentration for C is +0.015 mol, hence for A (which decreases by 0.015 mol) will be: 0.25 - 0.015 = 0.235 mol.
- For B, it will decrease by 2 times the amount of C due to the stoichiometry, i.e., 2 * 0.015 = 0.030 mol.
- Therefore, the amount of B will be: 0.25 - 0.030 = 0.220 mol.
Thus,
- Amount of A = 0.235 mol
- Amount of B = 0.220 mol.
Step 2
Calculate the value of the equilibrium constant Kc.
Answer
From the question, it states that:
- C = 0.020 mol
- B = 0.30 mol
- Volume of solution = 350 cm³ = 0.35 dm³.
First, we determine concentrations at equilibrium:
For C: ( ext{Concentration of C} = \frac{0.020}{0.35} = 0.0571 ext{ mol dm⁻³})
For B: ( ext{Concentration of B} = \frac{0.30}{0.35} = 0.8571 ext{ mol dm⁻³})
Then we use the expression for Kc for the reaction:
Step 3
Step 4
Suggest why an expression for K can be written without the concentration of water.
Answer
In aqueous solutions, the concentration of water is usually much greater than that of the other reactants. Thus, it can be considered effectively constant in the equilibrium expression because it doesn't significantly change. This assumption simplifies the reaction expression.
Step 5
Calculate the equilibrium concentration, in mol dm⁻³, of ClCH₂(CHOH)₂.
Answer
Given K = 37.0 and the concentration of ClCH₂CHO (which can be calculated):
- Initial amount of ClCH₂CHO = ( \frac{4.71 g}{78.5 g/mol} \approx 0.060 \text{ mol} ).
- Therefore, the equilibrium concentration for ClCH₂(COH)₂ can be calculated:
Step 6
Complete the mechanism in Figure 6 by adding two curly arrows, all relevant charges and any lone pairs of electrons involved.
Answer
- A curly arrow from the lone pair of the oxygen on water to the carbonyl carbon in ClCH₂CHO, creating a tetrahedral intermediate.
- A curly arrow from the bond between the carbonyl carbon and the pi bond to re-establish the double bond with the oxygen, producing ClCH₂(OH) and moving the electron density onto the oxygen.
Step 7
Suggest why this reaction is slower than the reaction in Question 09.5.
Answer
The reaction of water with ethanal is slower due to the electron-donating effect of the alkyl group, which makes the carbonyl carbon less electrophilic compared to chloroethanal. Hence, this results in a decreased rate of nucleophilic attack by water.